Nowadays, head-up displays (HUD) are becoming more and more popular because of their practical ergonomic designs in view of luminance/contrast, field of view, variable focal length, and so on. Nevertheless, the HUDs that are current available on the market are generally designed to use conventional lens sets or light-emitting diode (LED) segment displays to achieve the effect of image projection and field-of-view magnification.
Despite their low cost and ease to install, the HUDs which form simple patterns by the use of LED segment displays and then project the simple patterns on windshield glasses can easily cause eye fatigue to users since the displayed pattern are closely attached on the windshield glasses that the users will need to adjust their focal point from the far pavement to the near windshield glass.
The combination of projector and magnifier lens set is adopted and good for enable a user to focus on a remote object by bare eye. However, as it is known that a potential cause of visual fatigue in driving is repeated changes between near and far viewing, and as the projection of the virtual image that is produced by the use of the aforesaid projector combination may not be adjustable in response to vehicle speed change, visual fatigue to the driver can easily be triggered. Although such required focal length adjustability can be achieved by the use of lens set with specifically designed structure or by the change of curvature, the consequence is that the projector with such complicated curve surface is difficult to manufacture, and thus the yield rate of such projector is poor. In an other word, although the monitor using the aforesaid combination of projector and magnifier lens set can have farther range of display and better display effect, driver of a vehicle that may not be maintaining at a constant speed will still have to change his/her focus constantly and continuously while viewing the monitor.
Therefore, it is in need of a head-up display with variable focal length that can enable all the important driving information to be visible to the driver without blind spot, and can enable the position of projection to be determined according to the essentiality of data and vehicle speed.